The auditory nerve (AN) transmits all auditory information from the cochlea to the brain. In the cochlear nucleus (CN), AN fibers bifurcate to innervate multiple cell populations, including bushy cells (BCs) in the ventral CN, and fusiform cells (FCs) in the dorsal CN. These two cell types differ significantly in their ability to encode temporal properties of sound stimuli. BCs project to binaural circuits in the superior olivary complex and encode spectral and temporal characteristics that allow sounds to be localized in the horizontal plane. FCs project to monaural circuits in the inferior colliculus and detect spectrl cues for localizing sounds in the vertical plane. AN synapses on BCs and FCs are both glutamatergic and involve AMPARs as major postsynaptic glutamate receptors. At AN-BC synapses, synaptic transmission is extremely fast and reliable to preserve information contained in the timing of AN spikes. At AN-FC synapses, synaptic transmission is significantly slower than at AN-BC synapses. Understanding the synaptic mechanisms that make AN-BC synapses faster than AN-FC synapses has been an important question that has been intensely studied. However, which specific AMPAR subunits actually mediate fast synaptic transmission at AN synapses is still unresolved. The goal of the proposed studies is to provide understanding of the functional role of GluA3 AMPAR subunits at AN synapses on brainstem neurons and the sensitivity of AN synapses to auditory experience. Data obtained from this proposal will advance understanding of the cellular mechanisms underlying the temporal precision of sound coding in the normal and in the hearing impaired. Thus in Aim 1 we will test the hypothesis that GluA3 in AN-BC synapses is the AMPAR subunit that determines fast AMPAR kinetics. Aim 2 will test the hypothesis that increase in expression and localization within the PSD of GluA3 AMPAR subunits mediates the experience-dependent plasticity of AN-BC and AN-FC synapses. To achieve these goals, we will combine hearing tests (auditory brainstem responses, ABRs) to analyze the ability of the brainstem to respond to sound stimuli in vivo, quantitative ultrastructural and molecular techniques, genetic approaches (knockouts) and electrophysiology in acute brainstem slices of adult normal hearing and monaurally earplugged mice. Specifically, we will use freeze-fracture and postembedding immunogold labeling, qRT-PCR together with whole-cell recording to identify morphological, molecular and functional alterations at AN synapses. The results of our studies can be applied to efforts to optimize strategies for treating hearing loss and other hearing disorders. A large body of evidence indicates that the auditory system is highly specialized. Systematic, rigorous studies of the synaptic mechanisms underlying the specializations will both suggest and inform rational therapeutic approaches.